Thursday, September 5, 2013

Liquid Chromatography: Separating Plant Pigments

In this week's book, Gianna had to identify lots of leaves for a school project. We're going to learn about leaves too with this awesome chromatography experiment by ecologist Heather Hawke. Take it away, Heather!

Have you ever wondered why leaves turn color in the fall? They don’t really switch colors – some of those yellows and oranges are there all along!

Plants need food just like we do, only thing is, they make their own. They use a type of molecule, pigments, to capture sunlight. The plants can then use that energy to make sugar from carbon dioxide and water (photosynthesis).

Each kind of pigment works best at certain wavelengths (the different colors in a rainbow). The most common pigment group, chlorophyll, is really good at using most of the spectrum, except green. Then why are leaves green? That’s because chlorophylls soak up all the other colors. The green light bounces off, hits your eyes, and you see green.

Some pigments are better at absorbing other parts of the spectrum. The carotenoids give plants such as carrots or bananas bright yellows and oranges. The anthocyanins appear red to blue and are mostly present in flower petals and fruits such as cranberries or cherries. The uncommon betalains are found in only a few plants, but are responsible for red beets and colorful bougainvillea! Plants use a mixture of pigments in their leaves to capture as much of the energy in sunlight as possible.
In the fall, as the days get shorter, plants slowly stop making the chlorophylls and eventually their green fades away. Once the carotenoids are not swamped out anymore, their colors are finally revealed! Some plants will make anthocyanins as the days get shorter, and so will turn red.
In this experiment, you will try to find out how many pigments are in different leaves. You will separate the pigments using filter paper and rubbing alcohol, a solvent.
  • Filter paper (coffee filter paper works though high-quality filter paper gives the crispest results), cut into long rectangular strips
  • Rubbing alcohol Supervise young children! Rubbing alcohol is toxic if ingested!
  • A pencil
  • Tape
  • A quarter
  • A small, clear glass
  • Leaves – choose some you know will turn color in the Fall
1. Tape top of paper strip to a pencil. Balance pencil on top of glass. Trim bottom of strip so it is close to, but not touching the bottom of the glass.
2. Lay paper strip on flat surface. Place leaf near bottom of strip and rub a dark line onto the paper, parallel to the bottom of the strip (see photo). The neater and darker your line, the better your results will be.
3. Replace pencil on glass. Make sure the paper is hanging straight down. Carefully pour in the rubbing alcohol until it just touches the bottom of the paper, but does not cover your pigment line. Try not to splash the paper.
4. Observe over the next hour as the alcohol reaches the top of the strip. Remove the strip and lay flat to dry.
As the rubbing alcohol moves up the paper (by capillary action), it carries the pigment molecules. Some of the pigments are larger than others. The smallest ones travel fastest so are nearer the top. The bigger ones are closer to the bottom.
This chromatography strip is from a beet leaf. Notice the purple betalain pigment at the top.


1. How many pigments do you see (there can be different shades of the same type of pigment – for instance, “chlorophyll a” is almost teal-colored as compared to the darker “chlorophyll b.”

2. Which of your pigments are the largest?

3. Did you get different results from different types of leaves?

4. If you repeated this experiment every couple of weeks until the leaves drop, what would you expect to see?

5. If plants did not have pigments, and so could not make sugar, what would you eat? Think this one through.

*Fun tip. Plant pigments are important for us humans! For instance, the carotinoids are powerful antioxidants and help your eyes stay healthy (eat your carrots and tomatoes!).
Thank you so much, Heather! I can't wait to try this with my kids. And if you're looking for more fun science from Heather, check out this awesome Rainbow of Ants activity!

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